The present invention is generally related to the fields of magnetic resonance imaging (MRI) and magnetic resonance therapy (MRT).
MRI systems for performing whole body imaging usually employ large magnets which effectively surround the patient. Such magnets are usually large superconductor magnets which are expensive and difficult to maintain. MRI systems for performing local imaging of specific body parts or organs are known in the art.
U.S. patent application Ser. No. 08/898,773, now U.S. Pat. No. 5,900,793 to Katznelson et al., filed Jul. 23, 1997 and entitled xe2x80x9cPERMANENT MAGNET ASSEMBLIES FOR USE IN MEDICAL APPLICATIONSxe2x80x9d and incorporated herein by reference discloses, inter alia, compact permanent magnet assemblies for use in medical applications including MRI and/or MRT.
A typical application using an intra-operative MRI system is brain surgery. Reference is now made to FIG. 1 which is a schematic perspective view of a small organ dedicated MRI probe useful in brain surgery. The MRI probe 1 includes two annular permanent magnet assemblies 2 and 4 connected by a frame 3. The frame 3 and the magnet assemblies 2 and 4 are shaped for imaging the brain of a patient 6. During MRI assisted brain surgery or MRT, the head of the patient 6 is positioned between the two magnet assemblies 2 and 4. Reference is now made to FIG. 2 which is a schematic isometric view of the two permanent magnet assemblies 2 and 4 of FIG. 1. Each of the magnet assemblies 4 and 2 includes three preferably concentric annular permanent magnets 4a, 4b, 4c and 2a, 2b, 2c (not shown in drawing). The annular permanent magnets 4a, 4b and 4c are offset from each other along the axis 12, and the annular permanent magnets 2a, 2b and 2c (not shown) are also offset from each other along the axis, 12 as disclosed in U.S. patent application Ser. No. 08/898,773 to Katznelson et al. now U.S. Pat. No. 5,900,793.
The axis 12 is the axis of symmetry of both magnet assemblies 2 and 4, passing through their centers. The axis 12 coincides with the z-axis along which the main magnetic field generated by the magnet assemblies 2 and 4 is oriented.
In order to reduce eddy currents each one of the concentric annular permanent magnets 4a, 4b, 4c, 2a, 2b and 2c is formed from segments 24 each of which is permanently magnetized in a known manner and then attached to the neighboring segments using an electrically non-conducting glue (not shown) or non-conductive spacers (not shown). For example, the segments 24 can be made from a neodymium-iron-boron (Ndxe2x80x94Fexe2x80x94B) alloy. However, the segments 24 can be made from any other alloy or ceramic material suitable for forming permanent, magnets of sufficient magnetic field strength. Preferably, the material from which the segments 24 are made should have a relatively low electrical conductivity.
The magnet assemblies 2 and 4 joined together by frame 3 (not shown in FIG. 2) define a region 16 having therein a volume 18 of substantially uniform magnetic field, between the pair of magnet assemblies 2 and 4.
The MRI probe 1 further includes Gradient coils (not shown) for generating gradient fields, shim coils (not shown) for active shimming of the main magnetic field, RF coils (not shown), a temperature control system (not shown) and an RF shield (not shown).
Ordinarily, the gradient fields are generated by a set of coils, through which a current of an adequate magnitude flows. During the periods of building up and decay of the currents, the temporal change of the magnetic flux, originally generated by the currents, creates eddy currents in conductive materials situated in their vicinity such as soft iron parts or permanent magnet parts used in prior art MRI permanent magnets or the aluminum enclosures of the cooling systems used in super-conducting magnets of MRI systems. The eddy currents generated by the gradient coil magnetic flux changes, generate secondary magnetic fields which may interfere with the primary gradient fields and affect their precision in encoding the spatial information.
In prior art MRI devices, the gradient coils are located within the internal free volume situated in the main magnet, where the imaged body is also introduced. To attenuate the effect of the spurious eddy currents, prior art MRI devices may use shielded gradient coils or pre-emphasis circuits which modify gradient amplifier demand in order to compensate for eddy currents. In small organ dedicated MRI probes and in MRI probes adapted for intra-operative use such as the MRI probe 1 of FIG. 1, the dimensions of the region 16 (best seen in FIG. 2) for accommodating the organ to be imaged are limited by practical considerations. Generally, the design of such MRI systems involves a tradeoff between maximizing the intensity and homogeneity of the magnetic field in as large an imaging volume as possible and providing maximal accessibility of the surgeon to the organ undergoing surgery. For example, the MRI probe 1 (FIGS. 1 and 2) is designed to maximize the size of the volume 18 of homogenous magnetic field while keeping the size of the magnet assemblies 2 and 4 minimal while allowing enough space for positioning the shoulders of the patient 6. If one tries to increase the space available for the shoulders of the patient 6 by increasing the distance between the magnet assemblies 2 and 4 along the axis 12, the resulting decrease in the strength and homogeneity of the magnetic field will have to be compensated. The magnetic field can be compensated by, increasing the thickness of the annular permanent magnets 4a, 4b, 4c of FIG. 2 and 2a, 2b and 2c (not shown in FIG. 2).
Increasing the thickness of the annular permanent magnets 4a, 4b, 4c, 2a, 2b and 2c (not shown) is practically limited since their magnetic field, depends non-linearly on their thickness. Thus, increasing the thickness of an annular permanent magnet above a certain value, results in a negligible contribution to the magnetic field strength.
The magnetic field can also be compensated by increasing the size and diameter of the magnet assemblies 2 and 4. However, increasing the diameter of the magnet assemblies 2 and 4 may in turn shift the location of the volume 18 relative to the desired position of the head of the patient 6. The shifting may also prevent access to and imaging of the lower part of the brain, affecting the types of surgery that can be performed using the probe 1.
Thus, placing the gradient coils and/or shim and RF coils within the already restricted region 16 between the magnet assemblies 2 and 4, limits even further the space available for positioning the organ to be imaged and may hinder access to the organ undergoing surgery and the placing and manipulating of surgical instruments within that organ during surgery.
Furthermore, in MRI systems using permanent magnets, if the gradient coils are positioned in close proximity to the permanent magnets, the heat developed in the resistive gradient coils by the currents flowing within the coils may heat the permanent magnet. The heat generated by the gradient coils may thus cause local temperature increase in the permanent magnets. Such temperature changes are undesirable since the field generated by permanent magnets is highly susceptible to large variations induced by local temperature changes.
MRI systems based on permanent magnets such as the MRI probe 1 of FIG. 1 or the MRI probe of FIG. 2, do not include electrically conducting structures operating as magnetic flux return structures. This fact, in addition to the segmented structure of the annular permanent magnets 4a, 4b, 4c and 2a, 2b and 2c (not shown) and the intrinsic low conductivity of the Ndxe2x80x94Fexe2x80x94B alloy from which they are made, substantially reduce the spurious eddy current problem.
Whole body MRI/MRT systems typically use a fixed installation RF cage for preventing magnetic, electromagnetic and electrical noise from the outside from penetrating into the imaging volume inside the probe and interfering with the weak NMR signals generated during imaging. In addition, the RF cage is also used to reduce the leakage of the RF radiation generated within the probe during imaging to prevent disturbances to other electrical devices used near the MRI probe.
Unfortunately, for practical reasons, large fixed installation RF cages or RF rooms cannot always be used with small organ dedicated MRI or MRT probes of the type used for intra-operative imaging such as the MRI probe 1 of FIG. 1. For example, while the small organ dedicated MRI probe 1 may be operated within a large shielded RF room, this will necessitate the use of special expensive shielded surgical equipment that is designed to create minimal RFI disturbances so as not to interfere with the operation of the MRI probe 1.
There is therefore provided, in accordance with a preferred embodiment of the present invention, electromagnetic apparatus for use in an MRI device. The probe includes a first permanent magnet assembly having a first surface and a second surface thereof. The probe also includes a second permanent magnet assembly having a third surface and a fourth surface thereof. The second permanent magnet assembly opposes the first permanent magnet assembly such that the second surface and the third surface define an open region therebetween, for producing a predetermined volume of substantially uniform magnetic field extending in a first direction parallel to a first axis. The volume is disposed within the open region.
The probe also includes an energizable transmitting RF coil for producing an RF electromagnetic field within the volume, an energizable z-gradient coil for producing a magnetic field gradient extending within the open region in the first direction and parallel to the first axis, an energizable x-gradient coil for producing a magnetic field gradient extending within the open region in parallel to a second axis orthogonal to the first axis, and an energizable y-gradient coil for producing a magnetic field gradient extending within the open region in parallel to a third axis orthogonal to the first axis and the second axis. At least one of the x-gradient coil, y-gradient coil and z-gradient coil is positioned outside of the open region.
Furthermore, in accordance with another preferred embodiment of the present invention, the transmitting RF coil includes at least a first portion thereof positioned within the open region adjacent the second surface and at least a second portion thereof positioned within the open region adjacent the third surface. The first portion and the second portion of the transmitting RF coil are electrically connected in series.
Furthermore, in accordance with yet another preferred embodiment of the present invention, the transmitting RF coil further includes a third portion thereof including current return conductors positioned outside of the open region and adjacent the first surface, and at least a fourth portion thereof including current return conductors positioned outside of the open region and adjacent the fourth surface to increase the efficiency of the transmitting RF coil. The first portion, second portion, third portion and fourth portion of the transmitting RF coil are electrically connected in series.
Furthermore, in accordance with another preferred embodiment of the present invention, the apparatus further includes an energizable shim coil for improving the homogeneity of the substantially uniform magnetic field.
Furthermore, in accordance with another preferred embodiment of the present invention, the shim coil includes a first shim coil portion positioned outside of the open region and opposed to the first surface of the first permanent magnet assembly, and a second shim coil portion positioned outside of the open region and opposed to the fourth surface of the second permanent magnet assembly.
Further still, in accordance with another preferred embodiment of the present Invention, the first shim coil portion and the second shim coil portion are electrically connected in series.
Furthermore, in accordance with another preferred embodiment of the present invention, at least one of the x-gradient coil, y-gradient coil and z-gradient coil includes a first coil portion thereof opposed to the first surface of the first permanent magnet assembly and a second complementary coil portion thereof opposed to the fourth surface of the second permanent magnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, the first coil portion and the second coil portion of the at least one of the x-gradient coil, y-gradient coil and z-gradient coil are electrically connected in series.
Furthermore, in accordance with another preferred embodiment of the present invention, the first coil portion and the second coil portion of at least one of the x-gradient coil, y-gradient coil and z-gradient coil are substantially planar printed circuits, the first coil portion is assembled into a first multi-layer printed circuit assembly opposed to the first surface, and the second coil portion is assembled into a second multi-layer printed circuit assembly opposed to the fourth surface.
Furthermore, in accordance with another preferred embodiment of the present invention, each of the first multi-layer printed circuit assembly and second multi-layer printed circuit assembly further includes a portion of an energizable shim coil, the portion of the shim coil is a substantially planar printed circuit.
Furthermore, in accordance with another preferred embodiment of the present invention, the apparatus further includes a mounting of low permeability material for mounting the first permanent magnet assembly and the second permanent magnet assembly in opposition to each other.
Furthermore, in accordance with another preferred embodiment of the present invention, the first permanent magnet assembly includes a first annular permanent magnet with a first and a second surface thereof. The first surface of the first annular permanent magnet is of a first magnetic polarity and the second surface of the first annular permanent magnet is of a second magnetic polarity. The first annular permanent magnet has an inside diameter. The first annular permanent magnet has at least a portion of the first surface of the first annular magnet lying in a first plane to provide a first magnetic field in the open region. The first magnetic field has a zero rate of change in a first direction at a first point in the open region. The first magnet assembly also includes at least a second annular permanent magnet with a first and a second surface thereof. The first surface of the second annular magnet is of the first magnetic polarity and the second surface of the second annular permanent magnet is of the second magnetic polarity. The second annular permanent magnet has an outside diameter which is smaller than the inside diameter of the first annular permanent magnet, with at least a portion of the first surface of the second annular magnet lying in a second plane spaced from the first plane to provide a second magnetic field whereby the second magnetic field is superimposed upon the first magnetic field in the open region, having a zero rate of change in the first direction at a second point different from the first point. The second permanent magnet assembly includes a third annular permanent magnet with a first and a second surface thereof, the first surface of the third annular permanent magnet is of the second magnetic polarity and the second surface of the third annular permanent magnet is of the first magnetic polarity. The third annular permanent magnet has an inside diameter, the third annular permanent magnet has at least a portion of the first surface of the third annular magnet lying in a third plane to provide a third magnetic field, whereby the third magnetic field is superimposed on the first and second magnetic fields in the open region, having a zero rate of change in the first direction at a third point different from the first and second points. The second magnet assembly also includes at least a fourth annular permanent magnet having a first and a second surface thereof, the first surface of the fourth annular magnet is of the second magnetic polarity and the second surface of the fourth annular permanent magnet is of the first magnetic polarity. The fourth annular permanent magnet has an outside diameter which is smaller than the inside diameter of the third annular permanent magnet, with at least a portion of the first surface of the fourth annular permanent magnet lying in a fourth plane spaced from the third plane to provide a fourth magnetic field, whereby the fourth magnetic field is superimposed upon the first, second and third magnetic fields, in the open region, having a zero rate of change in the first direction at a fourth point different from the first, second and third points.
Furthermore, in accordance with another preferred embodiment of the present invention, the first axis passes through the centers of the first annular permanent magnet, the at least second annular permanent magnet, the third annular permanent magnet and the at least fourth annular permanent magnet.
Furthermore, in accordance with another preferred embodiment of the present invention, the first annular permanent magnet, the at least second annular permanent magnet, the third annular permanent magnet and the at least fourth annular permanent magnet are rare-earth permanent magnets.
Furthermore, in accordance with another preferred embodiment of the present invention, the rare-earth permanent magnets are neodymium-iron-boron alloy permanent magnets.
Furthermore, in accordance with another preferred embodiment of the present invention, at least one of the first annular permanent magnet, the at least second annular permanent magnet the third annular permanent magnet and the at least fourth annular permanent includes a plurality of segments attached to adjacent segments using an electrically non-conductive adhesive.
Furthermore, in accordance with another preferred embodiment of the present invention, the segments are equiangular segments.
Furthermore, in accordance with another preferred embodiment of the present invention, the segments have a trapezoidal cross-section in a plane orthogonal to the first direction.
Furthermore, in accordance with another preferred embodiment of the present invention, the z-gradient coil includes a first gradient coil portion concentrically disposed between the first annular permanent magnet and the at least second annular permanent magnet, and a second gradient coil portion concentrically disposed between the third annular permanent magnet and the at least fourth annular permanent magnet. The first and second gradient coil portions have their longitudinal axes coincident with the first axis.
Furthermore, in accordance with another preferred embodiment of the present invention, the apparatus further including at least one receiving RF coil placeable adjacent to an organ or body part disposed within the open region.
Furthermore, in accordance with another preferred embodiment of the present invention, the transmitting RF coil is a linearly polarizing RF coil.
Furthermore, in accordance with another preferred embodiment of the present inventions the transmitting RF coil is a circularly polarizing RF coil.
Furthermore, in accordance with another preferred embodiment of the present invention, the circularly polarizing RF coil is a quadrature-hybrid RF coil.
Furthermore, in accordance with another preferred embodiment of the present invention, the first permanent magnet assembly includes a first plurality of nested polygonally or elliptically shaped annular permanent magnets, and the second permanent magnet assembly includes a second plurality of nested polygonally or elliptically shaped annular permanent magnets the first plurality being opposed to the second plurality such that the second plurality is configured as a mirror image of the first plurality.
Furthermore, in accordance with another preferred embodiment of the present invention, at least one of the x-gradient coil, y-gradient coil and z-gradient coil is positioned below the first permanent magnet assembly and the second permanent magnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, the x-gradient coil, the y-gradient coil and the z-gradient coil are planar printed circuit coil boards assembled within a single multi-layer printed circuit assembly positioned underneath the first permanent magnet assembly and the second permanent magnet assembly.
There is further provided, in accordance with a preferred embodiment of the present invention, electromagnetic apparatus for use in an MRI device. The apparatus includes a permanent magnet assembly having at least a first surface defining a first side of the permanent magnet assembly and a second surface defining a second side of the permanent magnet assembly opposed to the first side, for producing a predetermined volume of substantially uniform magnetic field extending in a first direction beyond the first surface. The apparatus further includes an energizable transmitting RF coil for producing an RF electromagnetic field within the volume. At least a portion of the RF coil is positioned adjacent the first surface of the permanent magnet assembly. The apparatus also includes an energizable z -gradient coil for producing a magnetic field gradient extending within the volume in the first direction parallel to a first axis. The apparatus also includes an energizable x-gradient coil for producing a magnetic field gradient extending within the volume parallel to a second axis orthogonal to the first axis. The apparatus also includes an energizable y-gradient coil for producing a magnetic field gradient extending within the volume parallel to a third axis orthogonal to the first axis and to the second axis. At least one of the x-gradient coil, y-gradient coil and z-gradient coil is positioned opposing the second surface of the permanent magnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, the apparatus further includes at least one energizable shim coil for improving the homogeneity of the substantially uniform magnetic field.
Furthermore, in accordance with another preferred embodiment of the present invention, the at least one shim coil is a substantially planar coil opposing the second surface of the permanent magnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, the x-gradient coil, the y-gradient coil and the z-gradient coil are substantially planar printed circuits assembled within a substantially planar multi-layer printed circuit assembly. The multi-layer printed circuit assembly is disposed on the second side of the permanent magnet assembly facing the second surface.
Furthermore, in accordance with another preferred embodiment of the present invention, the multi-layer printed circuit assembly further includes at least one energizable shim coil. The at least one shim coil is a substantially planar printed circuit.
Furthermore, in accordance with another preferred embodiment of the present invention, the permanent magnet assembly includes a first annular permanent magnet having an upper and a lower surface thereof. The upper surface of the first annular permanent magnet is of a first magnetic polarity and the lower surface of the first annular permanent magnet is of a second magnetic polarity. The first annular permanent magnet has an inside diameter. The first permanent magnet has at least a portion of the upper surface of the first annular magnet lying in a first plane and providing a first magnetic field in the predetermined volume. The first magnetic field has a zero rate of change in the first direction at a first point. The permanent magnet assembly further includes at least a second annular permanent magnet having an upper and a lower surface thereof. The upper surface of the at least second annular permanent magnet is of the first magnetic polarity and the lower surface of the at least second annular permanent magnet is of the second magnetic polarity. The at least second annular permanent magnet has an outside diameter which is smaller than the inside diameter of the first annular permanent magnet. The at least second annular permanent magnet provides a second magnetic field. The permanent magnet assembly also includes low permeability material interconnecting the first annular permanent magnet with the second annular permanent magnet, so that at least a portion of the upper surface of the second annular permanent magnet is in a second plane spaced from the first plane. The second magnetic field is superimposed upon the first magnetic field, in the predetermined volume, having a zero rate of change in the first direction at a second point different from the first point.
Furthermore, in accordance with another preferred embodiment of the present invention, the first axis passes through the center points of the first annular permanent magnet and the at least second annular permanent magnet.
Furthermore, in accordance with another preferred, embodiment of the present invention, the first annular permanent magnet and the at least second annular permanent magnet are rare-earth permanent magnets.
Furthermore, in accordance with another preferred embodiment of the present invention, the rare-earth permanent magnets are neodymium-iron-boron alloy permanent magnets.
Furthermore, in accordance with another preferred embodiment of the present invention, at least one of the first annular permanent magnet and the at least second annular permanent magnet includes a plurality of segments attached to adjacent segments using an electrically non-conductive adhesive.
Furthermore, in accordance with another preferred embodiment of the present invention, the segments are equiangular segments.
Furthermore, in accordance with another preferred embodiment of the present invention, the segments have a trapezoidal cross-section in a plane orthogonal to the first direction.
Furthermore, in accordance with another preferred embodiment of the present invention, the z-gradient coil is an extended gradient coil concentrically disposed between the first annular permanent magnet and the at least second annular permanent magnet, the z-gradient coil has a longitudinal axis coincident with the first axis.
Furthermore, in accordance with another preferred embodiment of the present invention, the apparatus further includes at least one receiving RF coil positioned on the first side of the permanent magnet assembly and placeable adjacent to an organ or body part to be imaged using the apparatus.
Furthermore, in accordance with Another preferred embodiment of the present invention, the transmitting RF coil is a linearly polarizing RF coil.
Furthermore, in accordance with another preferred embodiment of the present invention, the transmitting RF coil is a circularly polarizing RF coil.
Furthermore, in accordance with another preferred embodiment of the present invention, at least a portion of the transmitting RF coil is positioned on the second side of the permanent magnet assembly opposing the second surface of the permanent magnet assembly to improve the efficiency of the transmitting RF coil.
There is also provided, in accordance with another preferred embodiment of the present invention, electromagnetic apparatus for use in an MRI device. The apparatus includes a permanent magnet assembly having a first surface and a second surface for producing a predetermined volume having a magnetic field varying substantially linearly along a first axis. The volume extends in a first direction beyond the first surface along the first axis. The magnetic field is substantially uniform In any plane which is included within the predetermined volume and which is orthogonal to the first direction within the predetermined volume. The apparatus further includes an energizable transmitting RF coil for transmitting RF radiation. The RF coil has at least one portion thereof positioned opposing the first surface of the permanent magnet assembly. The apparatus also includes an energizable x-gradient coil for producing a magnetic field gradient along a second axis orthogonal to the first axis. The apparatus also includes an energizable y-gradient coil for producing a magnetic field gradient along a third axis orthogonal to the first axis and to the second axis. At least one of the x-gradient coil and y-gradient coil is positioned opposing the second surface of the permanent magnet assembly.
Furthermore, in accordance with another preferred embodiment of the present invention, the apparatus further includes at least one receiving RF coil positioned on the first side of the permanent magnet assembly and placeable adjacent to an organ or body part to be imaged using the apparatus.
There is also provided, in accordance with another preferred embodiment of the present invention, a method for constructing electromagnetic apparatus for use in an MRI device. The method includes the steps of providing a first permanent magnet assembly having a first surface and a second surface thereof, providing a second permanent magnet assembly having a third surface and a fourth surface thereof, positioning the second permanent magnet assembly opposite the first permanent magnet assembly such that the second surface and the third surface define an open region therebetween, for producing a predetermined volume of substantially uniform magnetic field extending in a first direction parallel to a first axis, the volume is disposed within the open region, providing an energizable transmitting RF coil for producing an RF electromagnetic field within the volume, providing an energizable z -gradient coil for producing a magnetic field gradient extending within the open region in the first direction and parallel to the first axis, providing an energizable x-gradient coil for producing a magnetic field gradient extending within the open region in parallel to a second axis orthogonal to the first axis, providing an energizable y-gradient coil for producing a magnetic field gradient extending within the open region in parallel to a third axis orthogonal to the first axis and the second axis, providing at least one receiving RF coil placeable adjacent to an organ or body part to be imaged for receiving RF signals from the organ or body part, and positioning at least one of the x-gradient coil, y-gradient coil and z-gradient coil outside of the open region for reducing the loading of the transmitting RF coil and the at least one receiving RF coil by the at least one of the x-gradient coil, y-gradient coil and z-gradient coil.
There is further provided, in accordance with another preferred embodiment of the present invention, a method for constructing electromagnetic apparatus for use in an MRI device. The method includes the steps of providing a permanent magnet assembly having at least a first surface defining a first side of the permanent magnet assembly and a second surface defining a second side of the permanent magnet assembly opposed to the first side, for producing a predetermined volume of substantially uniform magnetic field extending in a first direction beyond the first surface, providing an energizable transmitting RF coil for producing an RF electromagnetic field within the volume, positioning at least a portion of the transmitting RF coil adjacent the first surface of the permanent magnet assembly, providing at least one receiving RF coil placeable adjacent to an organ or body part to be imaged for receiving RF signals from the organ or body part, providing an energizable z -gradient coil for producing a magnetic field gradient extending within the volume in the first direction parallel to a first axis, providing an energizable x-gradient coil for producing a magnetic field gradient extending within the volume parallel to a second axis orthogonal to the first axis, providing an energizable y-gradient coil for producing a magnetic field gradient extending within the volume parallel to a third axis orthogonal to the first axis and to the second axis, and positioning at least one of the x-gradient coil, y-gradient coil and z-gradient coil opposite the second surface of the permanent magnet assembly for reducing the loading of the transmitting RF coil and the at least one receiving RF coil by the at least one of the x-gradient coil, y-gradient coil and z-gradient coil.
Finally, there is provided, in accordance with another preferred embodiment of the present invention, a method for constructing electromagnetic apparatus for use in an MRI device. The method includes the steps of providing a permanent magnet assembly having a first surface and a second surface for producing a predetermined volume having a magnetic field varying substantially linearly along a first axis, the volume extends in a first direction beyond the first surface along the first axis, the magnetic field is substantially uniform in any plane included within the predetermined volume and orthogonal to the first direction within the predetermined volume, providing an energizable transmitting RF coil for transmitting RF radiation, positioning the transmitting RF coil such that at least one portion thereof opposes the fist surface of the permanent magnet assembly, providing at least one receiving RF coil placeable adjacent to an organ or body part to be imaged for receiving RF signals from the organ or body part, providing an energizable x-gradient coil for producing a magnetic field gradient along a second axis orthogonal to the first axis, providing an energizable y-gradient coil for producing a magnetic field gradient along a third axis orthogonal to the first axis and to the second axis, and positioning at least one of the x-gradient coil and y-gradient coil opposite the second surface of the permanent magnet assembly for reducing the loading of the transmitting RF coil and the at least one receiving RF coil by the at least one of the x-gradient coil and y-gradient coil.